The system which is currently in standard worldwide use for guiding aircraft to a landing is termed ILS (Instrument Landing System). In view of the fact that landing is the most critical procedure encountered in normal flight, numerous attempts have been made to develop a backup, or monitoring system, for ILS. Such systems are termed ILM systems (Independent Landing Monitoring Systems).
The weather radar, either modified or unmodified and cooperating with ground-based reflectors or beacons, has frequently been suggested for implementing an ILM, especially since weather radar is required aboard all airlines for use when flying under bad weather (IFR) conditions. In addition, the weather radar operates at microwave frequencies, thus permitting the generation of narrow, and hence precise, guidance beams with relatively compact antennas.
This concept is taught in my U.S. Pat. No. 3,243,816 which uses the airborne weather radar with ground installed passive reflectors, or alternatively with radar beacons of the frequency shift type, to outline guidance paths for landing, or for landing monitoring purposes.
More recently, Assam in his U.S. Pat. No. 3,729,737 added a further teaching involving the detecting, by means of airborne radar, of plural tilted reflectors to generate glideslope guidance patterns for ILM purposes. Both of the above teachings, however, failed to recognize the problems caused by multipath signals reflected off the ground, which alternately cancel and reinforce the direct path signals from reflectors, thereby making the reflector echoes periodically disappear as viewed from an approaching aircraft.
The importance of this multipath problem has been little understood even by organizations presumably skilled in the art. Recently, for example, the FAA conducted flight tests seeking to detect for guidance purposes ground installed radar reflectors of large radar cross-section, and reported that it was not possible to reliably detect such reflectors since the echoes from the reflectors would alternately appear and disappear. This phenomena was blamed on poor reflectors construction, when in reality it was due to multipath reflections.
Gillard et al in U.S. Pat. No. 4,104,634, recognized the multipath problem and devised a reflector system suitable for ILM and other purposes. This system minimizes the multipath problem by utilizing the ground as an integral portion of each installed reflector.
In my co-pending patent application, Ser. No. 082,512, filed Oct. 9, 1979, I also recognize the multipath problem, and utilize a ground-based passive reflector system to establish a target of known radar cross-section at a known range for use during final approach to calibrate the weather detection and precipitation measuring capability of the radar, the multipath problem being eliminated by having guidance reflectors with vertical directivity adjusted to reject multipath signals reflected from the ground.
However, even though satisfactory solutions to the multipath problem have been found which insure that ground installed radar reflectors will provide targets with large and stable radar cross-sections over well defined angular limits, it is still necessary to be able to reliably distinguish echoes radiated by such installed reflectors from echoes emanating from natural targets, i.e. ground clutter.
In general, the prior art recognizes that echoes from installed passive reflectors can be more easily distinguished from ground clutter if such installed reflectors are arranged with a known spacing, i.e., position encoding, since natural targets do not normally exhibit recognizable coded spacings. In Assam's U.S. Pat. No. 3,729,737, for example, multiple reflectors are arranged with coded spacings along the runway.
The use of passive reflectors results in deficiencies for certain ILM applications, however. A basic disadvantage is caused by the fact that to achieve coding, the reflectors must be physically separated longitudinally along the runway by at least one pulse length, and preferably two. This is necessary so that, as viewed at the radar presentation, successive echoes from such reflectors will be separated in time and will not tend to combine and generate a scintillating target, i.e., a target whose intensity changes radically with minor changes in range of the viewing radar. Although certain modern weather radars of the RCA Primus 500 or Bendix RDR 1400 type have short pulse lengths of 0.5 microsecond, corresponding to a down-range target length of 250 ft., even these short pulse lengths require that four installed guidance reflectors, two each for glideslope and two each for localizer purposes, be separated by an overall longitudinal separation of 1000 ft. to 2000 ft.
While such reflector separation is not a problem at conventional airport runways, it is quite impractical at helicopter landing ports which are limited in area, for example, being of the order of 100 ft. or so in certain applications, e.g., oil rigs or building tops.
This deficiency can be minimized by decreasing the pulse length of the airborne radar, for example, a pulse length of 0.1 microsecond corresponding to a physical length of 50 ft. Such a pulse length is utilized in a Bendix airborne weather radar currently being built for the Coast Guard. Such a pulse length would permit overall reflector separations in an array of reflectors of 200 to 400 ft. A pulse length of 0.05 microsecond, currently in use on marine radars, would permit an overall reduction in reflector separation to 100 to 200 ft.
While such short pulse lengths provide some help in solving the problem of reflector installations at sites of restricted area, their use would require extensive modification of existing radars. It is therefore a preferred approach to devise a method whereby existing production radars, of the RCA Primus 500 or Bendix RD 1400 type for example, can be used virtually unmodified to provide an ILM capability for restricted landing areas.
A weather radar ILM system based on cooperation with active reflectors can be used to solve the problem at physically restricted sites. An active reflector returns energy at a frequency that is different from the radar frequency and hence the active reflector echoes can be distinguished from ground clutter echoes by virtue of suitable frequency change. Multiple active reflectors can be located virtually side by side at a physically restricted site and distinguished by their use of multiple different frequencies.
One such active reflector is shown in my Frequency Shift Reflector (FSR) U.S. Pat. No. 3,108,275 whose use in an ILM system is recognized in my above noted U.S. Pat. No. 3,243,816. In this FSR system, the returned frequency is shifted by a prescribed amount from the incoming frequency by modulation, and the returned energy level is directly related to the incoming energy level. In my U.S. Pat. No. 3,243,816 multiple FSR reflectors, all using the same frequency shift, were utilized. However, the reflectors themselves were individually identified in that patent by longitudinal separation, rather than by frequency separation as discussed above, since the longitudinal separation was indispensible to the guidance method employed.
Another useful active reflector system is a conventional beacon that detects the radar energy at one frequency and is triggered to re-radiate energy at another frequency. In this system the re-radiated energy and frequency are constant, and are not varied according to incoming energy and frequency. The above two mentioned radars, RCA Primus 500 and Bendix RDR 1400, have such a beacon capability provided by a separate receiver that is tuned to the frequency of the ground based beacon which is triggered by the transmitted pulse of the airborne radar. This beacon capability is included in the airborne radar design to facilitate homing on areas such as oil rigs, where such a beacon is installed.
Gendreu et al. in U.S. Pat. No. 4,103,300 suggests the use of a beacon system for ILM purposes. His patent covers two basic techniques. In one technique, the airborne weather radar makes airborne range and angle measurements to one or more beacons for guidance purposes. In the second technique, multiple ground installed beacons with directional antennas radiate signals having known directivity with respect to a guidance path. By comparing the airborne-measured relative intensities of such signals, guidance information is obtained. This is similar to the conventional ILS guidance beams, except that each of Gendreu's radiated lobes is transmitted at a different frequency. This is the way in which his ground based multiple guidance beacons are distinguished both from each other and from ground clutter returns, i.e., by use of multiple different frequencies instead of by their relative time positions in a predetermined sequence of pulses. This technique tends to require complex airborne implementation and wide receiver bandwidths, and cannot use a standard weather radar.
What is desired is an independent landing monitor system or landing system, using active reflectors, that can utilize existing weather radars and associated beacon capability with minimal modification to existing equipment. Such weather radar and beacon capability is inherent in the RCA Primus 500 or Bendix RDR-1400 type radars and can be used as the basis for adding precision landing guidance to the already existing approach guidance with minimal modifications or additions to such radars.